The technology of telephone communications has evolved over a period of time during which telephony voice communication has been of primary concern. Telephony voice communication has been traditionally provided via circuit switched telephone facilities. Circuit switched facilities are characterized in that a circuit or a communication path is separately dedicated to each active telephone conversation throughout the entire duration of the telephone conversation. More recently communication paths have been more economically provided by respectively assigned channels in a time division multiplex (TDM) telephone exchange.
It is only in the last twenty years or so that consideration and requirements for communication systems capable of carrying a rapidly growing volume of data communication has had any significant impact on the production of communications systems in general. In contrast to the circuit switched design philosophy of telephone voice communications systems, more economical data transmission systems are typically based on a packet switching design philosphy. Packet switching is characterized in that a circuit or communication path is exclusively committed to various of data transactions one after another. Each data transaction occupies the communication path for a time which is consistent with the volume of the data divided by the bandwidth of the communication path.
Synchronous communications are most efficiently handled by circuit switched facilities. Each synchronous communication occupies a communication path or channel for the full duration of the communication without regard to utilization of bandwidth. Asynchronous communications are most efficiently handled by packet switched facilities. Each asynchronous communication, sometimes referred to as a transaction, utilizes the full bandwidth of a circuit path for only as much time as data volume divided by the bandwidth requires. Asynchronous data transmitted via circuit switched facilities seldom utilize the available bandwidth. In packet switched facilities if traffic is present the full bandwidth is used. However because of the asynchronous nature of information transfer between ports in a packet switched system, attempts to use this type of system for voice i.e. synchronous information transfers, have resulted in relatively intricate and complicated solutions which typically exhibit lesser performance than is practically acceptable. An extensive summary of the capabilities and consequences of packet switching and various exemplary systems has been documented by Roy D. Rosner under the title of "Packet Switching Tomorrow's Communications Today" and published by Lifetime Learning Publications, a division of Wadsworth, Inc., in Belmont, California.
It is apparent that asynchronous data information is inefficiently communicated by circuit switched facilities. Furthermore holding times for asynchronous data transmission in a circuit switched facility can greatly exceed the typical duration of a voice telephone conversation. Thus extensive data traffic tends to seriously congest the typical circuit switched network. It is also apparent that currently available packet switching facilities are not a practical alternative to circuit switched facilities for voice-like information as receiving delays are typically too long, and even worse are inconsistent. Thus in both public and private communications systems circuit switched facilities are typically provided. Where the occasion warrants, packet switching is provided as a separate network exclusively for asynchronous data communcations.